Method and apparatus for reducing drag on a moving body

ABSTRACT

A method of and apparatus for reducing drag produced by relative air movement on a moving body are disclosed. The method includes the steps of modifying a rear air pressure behind a rear of the body such that the rear air pressure is increased, wherein the air pressure modifying step includes inletting an amount of air from a boundary flow around the body and forming a pressure shell behind the rear of the body, wherein the pressure shell forming step includes forming a large vortex behind the rear of the body by outletting the inlet air at the rear of the body in a plurality of small high-energy vortices.

This is a continuation of application Ser. No. 07/712,046, filed Jun. 7,1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatuses for reducingdrag, and more particularly to methods and apparatuses for reducing dragproduced by relative air movement on a moving body.

2. Description of Prior Art

Numerous means have been sought to improve the fuel-efficiency of movingbodies, and especially moving bluff bodies, by reducing theiraerodynamic drag. In the field of surface transportation, andparticularly in the long-haul trucking industry, even small improvementsin fuel efficiency can reduce annual operating costs significantly.Previous investigations of aerodynamic drag of tractor-trailer trucksresulted in widespread adoption of air deflectors mounted on tractorcabs, and wholly redesigned tractors that utilize aerodynamic fairingsto gradually increase the relatively small frontal area of tractors tomatch, and to blend smoothly with, the larger cross-section of typicaltrailers. Current air deflectors and fairings help guide the slipstreamaround the front of tractor-trailer trucks, and thereby reduceaerodynamic drag and improve fuel efficiency.

However, bluff bodies include generally flat rear ends, and the flatrear end of bluff bodies such as trailers is known to contributesignificantly to aerodynamic drag. Current bluff bodies suffer from asevere pressure gradient from their widest point to their rear, suchthat a boundary layer therearound becomes stalled very rapidly, near thewidest point. A stalled boundary layer causes flow to separate and abroad eddying wake to form downstream of the separation. The net resultis the creation of considerable aerodynamic drag.

Previous attempts to streamline moving bodies such as bluff bodies havebeen constrained by legal and practical considerations. Federal andstate regulations restrict the size of highway transport vehicles andinhibit conventional methods of streamlining because they limit thelength and width of "add-on" aerodynamic devices. Current legalrestrictions exclude devices that improve energy efficiency even if theyresult in only minor changes in overall dimension. Conventionalstreamlined afterbodies or "boat-tails" attached to the rear of trailerssuffer the disadvantages of significantly increasing physical dimensionsof the trailer and of interfering with loading and unloading.Additionally, when not in use, large retrofit "boat-tail" devicesrequire additional space either for storage or for trailer parking, andfurther, limit the number of trailers that can be loaded onto flat-bedrail cars. Given the choice, trailer manufacturers currently prefer todesign for maximum cargo capacity instead of minimum aerodynamic drag.

Therefore, a practical device is needed to reduce aerodynamic drag,particularly base pressure drag, from bluff bodies like tractor-trailertrucks. Such a device or apparatus needs to be in compliance withexisting regulations, i.e., it needs to be within the scope ofexclusionary clauses that permit only minor changes in the overalldimensions of tractor-trailer trucks. Further, the device or apparatusneeds to reduce drag without interfering with cargo capacity, withoutaltering current methods of loading and unloading, without requiringadditional parking or storage space, and without changing currentshipping practice.

Therefore, the present invention provides a method and apparatus forreducing the drag produced by relative air movement on a moving body,and especially a moving square-bodied bluff body. The present inventionprovides a method and apparatus for improving the fuel efficiency of amoving body, without significantly altering overall physical dimensions,without reducing cargo capacity, without significantly interfering withloading and unloading, without requiring substantially greater parkingspace, and without altering current shipping practices.

SUMMARY OF THE INVENTION

To overcome the problems of the prior art, the present inventionprovides a method of reducing drag produced by relative air movement ona moving body, which includes the steps of modifying a rear air pressurebehind a rear of the body such that the rear air pressure is generallynear a frontal air pressure produced on a front of the body by therelative air movement, wherein the air pressure modifying step includesinletting an amount of air from a boundary flow around the body, andforming a pressure shell behind the rear of the body, wherein thepressure shell forming step includes forming a large vortex behind therear of the body by outletting the inlet air at the rear of the body ina plurality of small high-energy vortices.

The present invention also provides an apparatus for reducing drag on amoving body, which includes a controller, at least one sensor forsensing a pressure and yaw angle of a relative air movement, wherein anoutput of the sensor is connected to the controller, at least one airinlet for inletting air, a plurality of thrust augmenters connected tothe air inlet, and at least one valve connected between the air inletand the plurality of thrust augmenters, wherein the controller isconnected to the valve to control a rate of air flow through the valvein response to the output of the sensor.

The present invention further provides an apparatus for reducing drag ona moving body, which includes inletting means for inletting an amount ofair from a boundary flow, and pressure shell forming means for forming apressure shell, wherein the pressure shell forming means includes smallvortex forming means for forming a plurality a small high-energyvortices which form a large vortex, the small vortex forming meansincluding outletting means for outletting the inlet air, and entrainingmeans for entraining a portion of the boundary flow with the inlet air.

The present invention also provides an apparatus for reducing drag on amoving body, which includes at least one inlet mounted on a forwardportion of body, a plurality of thrust augmenters mounted on a rearportion of the body, at least one duct mounted on the body andconnecting the inlet with the thrust augmenters, at least one valve inthe duct, a pair of sensors mounted on the body to sense a pressure anda yaw angle of a relative air movement on the body, and a controllerconnected to the sensors and the valve to control the valve and thus anair flow rate through the thrust augmenters, based on outputs of thesensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic view of fluid flow around a streamlined body;

FIGS. 1(b)-(c) are velocity and pressure profiles, respectively, of afluid particle intercepting the streamlined body of FIG. 1(a) at acenter of its longitudinal axis;

FIG. 2(a) is a schematic view of fluid flow around a conventional bluffbody;

FIGS. 2(b)-(c) are velocity and pressure profiles, respectively, of afluid particle intercepting the bluff body of FIG. 2(a) at a center ofits longitudinal axis;

FIGS. 3(a)-(d) are vector representations of relative winds resultingfrom the combined effects of various ambient winds and various apparentwinds developed by the forward motion of a moving body;

FIG. 4(a) is a schematic view of fluid flow around a conventional bluffbody resulting from a relative wind acting in a direction which isskewed from the longitudinal axis of the bluff body by a yaw angle;

FIGS. 4(b)-(c) are velocity and pressure profiles, respectively, offluid particles intercepting the bluff body of FIG. 4 (a);

FIG. 5 is a graph of the effect of ambient wind on the power required tomaintain a constant speed of a bluff body;

FIG. 6(a) is a perspective view of a first embodiment of the apparatusof the present invention fitted onto a conventional bluff body;

FIG. 6(b) is a perspective view of a second embodiment of the apparatusof the present invention fitted onto a conventional bluff body;

FIG. 6(c) is a detail view of a portion of FIG. 6(a);

FIG. 6(d) is a detail view of a portion of FIG. 6(b);

FIG. 7(a) is a rear view of the apparatus and the bluff body of FIG. 6(a);

FIG. 7(b) is a rear view of the apparatus and the bluff body of FIG. 6(b);

FIG. 7(c) is a detail view of a portion of FIG. 7(a);

FIG. 7 (d) is a detail view of a portion of FIG. 7(b);

FIG. 8(a) is a side view of the apparatus and the bluff body of FIG. 6(b);

FIG. 8(b) is an enlarged view of a portion of FIG. 8(a);

FIG. 8(c) is a side view of the apparatus and the bluff body of FIG. 6(a);

FIG. 8(d) is a detail view of a portion of FIG. 8 (b);

FIG. 8(e) is a detail view of a portion of FIG. 8(c);

FIG. 9 is a schematic view of a boundary layer control system and amicroprocessor control system;

FIG. 10(a) is a side view of a mechanical power supply system of thepresent invention;

FIG. 10(b) is a top view of the system of FIG. 10(a);

FIGS. 11(a)-(f) and 11(h)-(s) are schematic views of flow directors ofthe apparatus of FIG. 6(a);

FIG. 11(g) is a cross-sectional view along line A--A in FIG. 11(f);

FIG. 12(a) is a view similar to FIG. 8(d);

FIG. 12(b) is a cross-sectional view along line B--B in FIG. 12(a);

FIG. 12(c) is a cross-sectional view along line C--C in FIG. 12(a);

FIG. 12(d) is a cross-sectional view along line D--D in FIG. 12(b);

FIG. 13(a) is a view similar to FIG. 7(a), showing the formation of aplurality of small vortices, and a large vortex;

FIG. 13 (b) is a view similar to FIG. 7(b), showing the formation of aplurality of small vortices, and a large vortex;

FIG. 13(c) is a view similar to FIG. 8(a), showing the creation of a"pressure shell" in the wake of the bluff body;

FIG. 13(d) is a schematic view of fluid flow around a bluff body onwhich the apparatus of the present invention is mounted;

FIG. 14(a) is a side view of an air inlet of the apparatus of thepresent invention;

FIG. 14(b) is a cross-sectional view along line E--E of FIG. 14(a).

DETAILED DESCRIPTION

FIG. 1(a) illustrates a streamlined body (2) such as a symmetric airfoilwith a leading edge (4) and a trailing edge (6). Streamlines (8) providevisualization of the fluid flow around streamlined body (2). Referringto FIGS. 1(b) and 1(c), a fluid particle intercepting streamlined body(2) at its longitudinal center and leading edge (4) experiences adeceleration from free-stream velocity (10) to zero at leading edge (4)of streamlined body (2). Concurrently, the dynamic pressure increasesfrom free-stream static pressure (12) and reaches a maximum "stagnationpressure" (14). As the fluid particle continues to move downstream, itaccelerates to above free-stream velocity and reaches a maximum speed(16) near the widest point (21) of streamlined body (2) with anassociated minimum pressure (18) that is less than free-stream staticpressure (12).

In an ideal streamlined body (2), as the fluid particle continuesdownstream, it decelerates to zero velocity at the trailing edge (6),where pressure once again reaches a maximum "stagnation pressure" (14).As the fluid particle recovers from the passage of streamlined body (2),it accelerates and reaches equilibrium with the free-stream velocity(10). Simultaneously, the pressure decays from "stagnation pressure"(14) and again reaches equilibrium with free-stream static pressure(12).

FIG. 2(a) illustrates a bluff body (20) with a maximum width at (21),and with a bluff trailing edge (22). Flow streamlines (8) are shownaround the bluff body (20). Due to the bluff trailing edge (22), thevelocity of a fluid particle in the stream does not reach zero at thetrailing edge (22), as indicated by the base velocity (24) in FIG. 2(b),and the pressure recovery is incomplete, as indicated by the basepressure (26) in FIG. 2(c). A comparison of FIGS. 1(b) and 2(b), and ofFIGS. 1(c) and 2(c) illustrates the difference between the velocity andpressure profiles created by the streamlined body (2) and the bluff body(20). In the bluff body (20), a difference between the stagnationpressure (14) at the leading edge (4) of the bluff body (20) and thebase pressure (26) at the bluff trailing edge (22) gives rise to aretarding force, i.e., base pressure drag. Flow in the wake of the bluffbody (20) is chaotic and is indicated as a turbulent wake (28) in FIG.2(a).

FIGS. 3(a)-(d) illustrate the variable nature of winds to which bothstreamlined and bluff bodies are subjected. As either the streamlinedbody (2) or the bluff body (20) moves through a stationary fluid, itsforward motion creates an apparent wind (30) from the point of referenceof the moving body. A vector representing apparent wind (30) is equal inmagnitude to the forward velocity of the moving body and exactlyopposite in direction. In the case where the moving body encounters anambient wind (32) at a separation angle S (33), the resulting relativewind (34) is a vector sum of the apparent wind (30) and the ambient wind(32). FIG. 3(a) illustrates a relative wind (34) created when an ambientwind (32) is a headwind and the separation angle β (33) equals zero.FIG. 3(b) illustrates a relative wind (34) when an ambient wind (32) isa crosswind and the separation angle β (33) equals 90 degrees. FIG. 3(c)illustrates a relative wind (34) when an ambient wind (32) isquartering, between a headwind and crosswind, and the separation angle β(33) is between zero and 90 degrees. FIG. 3(d) illustrates a relativewind (34) when an ambient wind (32) is a partial tailwind, and theseparation angle β (33) is between 90 and 180 degrees.

FIG. 4(a) illustrates a bluff body (20) such as a conventionaltractor-trailer truck with a bluff trailing edge (22), and with flowstreamlines (8) therearound resulting from a relative wind (34). In FIG.4(a), a relative wind (34) is skewed from the longitudinal axis (36) ofthe bluff body (20) by a yaw angle Δ (40). The velocity profile of FIG.4(b) and the pressure profile of FIG. 4(c) indicate different behaviorsin the flows near the upstream and downstream surfaces of the bluff body(20), of fluid particles which intercept the bluff body (20) at a point(17) of maximum stagnation pressure. The point (17) of maximumstagnation pressure is shifted from the leading edge (4) in FIG. 4(a)due to the yaw angle Δ (40) at which the relative wind (34) acts.

On the upstream side (42) of the bluff body (20), the velocity profileof FIG. 4(b) shows that a fluid particle first decelerates from afree-stream velocity (10) towards zero where reaches an intermediateoff-axis nose velocity (11) at the leading edge (4) and where iteventually reaches zero velocity at the point (17) on the bluff body(20). The fluid particle then accelerates gradually, as shown by thecurve (201), from zero velocity at the point (17) to a maximum basevelocity (24) near the trailing edge (22), and then graduallydecelerates once again until it reaches equilibrium with the free-streamvelocity (10).

Corresponding to the velocity profile of FIG. 4(b), the pressure profileof FIG. 4(c) indicates a rapid rise from a free-stream static pressure(12) towards a maximum stagnation pressure (14) at the point (17) and anintermediate off-axis nose pressure (15) is reached at the leading edge(4). The pressure then drops gradually on the upstream side (42) of thebluff body (20), as shown by the curve (202), to a minimum value, i.e.,base pressure (26), near the trailing edge (22). The pressure thereaftergradually rises to the value of the free-stream static pressure (12),and again reaches equilibrium. On the upstream side (42), the differencebetween the off-axis nose pressure (15) and the base pressure (26)reveals that pressure recovery is incomplete and that a retarding force,i.e., base pressure drag, is created.

On the downstream side (44) of the bluff body (20), the velocity profileof FIG. 4(b) shows that a fluid particle first decelerates fromfree-stream velocity (10) towards zero until it attains the intermediateoff-axis nose velocity (11) at the leading edge (4). The fluid particlethen accelerates rapidly, as shown by the curve (203), from the leadingedge (4) to a maximum velocity (23), and then decelerates slightly tothe base velocity (24) at the trailing edge (22). The fluid particlethen gradually decelerates to the free-stream velocity (10) whereby thefluid particle once again reaches equilibrium.

Corresponding to the velocity profile of FIG. 4 (b), the pressureprofile of FIG. 4 (c) indicates that a fluid particle experiences arapid pressure rise from the free-stream static pressure (12) towardsthe maximum stagnation pressure (14) until it reaches the intermediateoff-axis nose pressure (15) at the leading edge (4). The pressure thendecays rapidly on the downstream side (44) of the bluff body (20), asshown by the curve (204), to a minimum value (25) near the maximum width(21) of bluff body (20), rises slowly towards the free-stream staticpressure (12), but then drops to the value of the base pressure (26).The fluid particle then gradually recovers to the value of thefree-stream static pressure (12) whereby it achieves equilibrium. On thedownstream side (44) of the bluff body (20), the difference between thenose pressure (15) at the leading edge (4) on the bluff body (20) andthe base pressure (26) near the trailing edge (22) also indicates thatpressure recovery is incomplete and that base-pressure drag is created.Flow in the broad wake of bluff body (20) is chaotic and is indicated asthe turbulent wake (28).

FIG. 5 shows the effect of ambient winds on the power required toovercome aerodynamic drag while maintaining a constant speed as afunction of the separation angle β (33) between an ambient wind (32) andthe longitudinal axis (36) of a bluff body (20). The power required tomaintain constant speed for calm conditions is indicated as a baseline(60), and for ambient winds of 10 miles per hour as a curve (62). Thecurve (62) shows that the power requirement exceeds the baseline (60) bya factor of more than 1.5 when the ambient wind (32) intercepts thebluff body (20) at a separation angle β (33) between about 30 and 60degrees. For ambient winds of 20 miles per hour, a curve (64) indicatesthat a power requirement of nearly 2.5 times greater than the baseline(60) is required for certain separation angles β (33). FIG. 5 clearlyillustrates the need for drag reduction on bluff bodies. FIG. 5 isadapted from a report to the RANN Program, National Science Foundation,by Charles L. Brunow, "An Evaluation of Truck Aerodynamic Drag ReductionDevices and Tests," June, 1975, pp.106, which is hereby incorporatedherein by reference.

FIGS. 6(a)-8(e) provide a general overview of the preferred embodimentsof the invention installed on a bluff body (20), as represented by aconventional tractor-trailer truck having a bluff trailing edge (22).FIGS. 6(a)-8(e) illustrate a trailer fitted with air inlets (66) and airoutlets (70), and show details of static flow controllers in FIGS. 6(a), 6(c) , 7(a) , 7(c), 8(c), and 8(e), and of dynamic flow controllers inFIGS. 6(b), 6(d), 7(b), 7(d), 8(a), 8(b), and 8(d). FIGS. 7(a)-(d) arerear views of the apparatuses and the bluff body (20) of FIGS.6(a)-6(d), and FIGS. 8(a)-8(e) are side views of the apparatuses and thebluff body (20).

The outlets (70), the static flow controllers, and the dynamic flowcontrollers will be described hereinbelow in greater detail. The inlets(66), as can be seen in FIGS. 6(a), 6(b), and 8(a), may extend aroundthe top and sides of forward portions of the bluff body (20), and allowair from a boundary flow around the bluff body (20) to be inlet. As seenin FIGS. 6(a)-6(b) and 14(a), the inlets (66) include slots (210)therein through which air is inlet. The slots (210) open into aplurality of chambers (212) which are divided by internal baffles (214).The internal baffles (214) are used to equalize a rate of intake flowalong the length of an inlet (66), and any number of baffles (214) maybe used to achieve this end. As shown in FIGS. 6(a) and (b), thechambers (212) extend parallel to each other along the length of aninlet (66), and preferably all extend to a lower end of the inlet (66)to connect with ducts (68) of a boundary layer control system which willbe described hereinbelow. It should be noted that the relative lengthsof the slots (210) may be varied to help equalize the rate of intakeflow along the length of an inlet (66).

A boundary layer control system is shown in FIG. 9, and includes airinlets (66) which control the growth of a boundary layer by capturing aportion of boundary layer flow (65), the air outlets (70), and ducts(68) which connect the air inlets (66) to the air outlets (70), whichare located around the rear periphery of the bluff body (20) near thebluff trailing edge (22). A blower (72) is located in the ductwork (68)in communication with both the air inlets (66) and the air outlets (70),and it provides a variable pressure differential between the inlets andthe outlets. The blower (72) responds to signals from a microprocessor(74) which controls the intensity and operation of the blower (72) aswell as the operation of valves (80) in the ductwork (68). The airoutlets (70) may be single or multiple and may be "vectored" via controlsignals from the microprocessor (74) so as to respond to differentambient conditions and relative winds (34) for the purpose of minimizingaerodynamic drag, as will be described further hereinbelow.

During operation, the air inlets (66) capture a variable portion of theboundary layer flow (65) from around forward portions of the bluff body(20), and, via the ductwork (68), the valves (80), and the blower (72),allow the inlet air to pass through air outlets (70) located in regionsof relatively low base pressure (26) near the bluff trailing edge (22).As will be described in more detail hereinbelow, the additional mass ofair released behind the bluff body (20) acts to increase the basepressure (26) and to generate an invisible streamlined afterbody (94),shown in FIG. 13(c), that is essentially a "pressure shell". Asillustrated by FIG. 13(d), the streamlined afterbody (94) preventschaotic flow in turbulent wake (28) and thereby decreases base pressuredrag.

A microprocessor control system is also shown in FIG. 9, and includesthe microprocessor (74), pressure transducers (76), control wiring (78),and the air valves (80). The microprocessor (74) controls and regulatesthe performance of the boundary layer control system over a full rangeof road conditions where an ambient wind (32) and an apparent wind (30)combine to yield a resulting relative wind (34) which acts at a yawangle Δ (40) which is angularly displaced from the longitudinal axis(36) of the bluff body (20), such as shown in FIG. 4(a). The relativewind (34) is highly variable and the value of the yaw angle Δ (40)changes with each change in local wind direction and speed as well aswith each change of vehicle direction and speed. Also, the relative wind(34) creates greater or lesser separation of the boundary layer, andproportionately greater or lesser aerodynamic drag, depending upon thevalue of the ambient wind (32) as illustrated by FIG. 5.

The pressure transducers (76) are located on the bluff body (20) as seenin FIGS. 6(a)and (b), and they sense the pressure and the yaw angle Δ(40), and send an output via the control wiring (78) to themicroprocessor (74). The microprocessor (74) responds to the transducersoutput by controlling the valves (80) and thus controlling suction ratesat the various air inlets (66) and blowing rates at the various airoutlets (70). By operation of the microprocessor (74), the effectivenessof the boundary layer control system is optimized throughout a broadrange of ambient conditions, including a highly variable relative wind(34). Air may be routed under the control of microprocessor (74) to airoutlets (70) in greater or lesser volume, and differentially from theupstream side (42) to the downstream side (44) of the bluff body (20),in response to, for example, the different upstream and downstream flowconditions shown in FIG. 4(a).

As shown in FIG. 4(c), the pressure on the upstream side (42) of thebluff body (20) is greater than the pressure on the downstream side (44)of the bluff body (20). To help equalize the pressure between theupstream and the downstream sides, the microprocessor (74) can allow theinlet valves (80) on the upstream side (42) to open further than theinlet valves (80) on the downstream side (44), to thus remove a greaterportion of the boundary layer flow on the upstream side (42), and tothus outlet a greater amount of air out of the outlets (70) on thedownstream side (4), to help equalize the pressure on the upstream side(42). Also, the apparatus may be designed such that the amount of airwhich is allowed to pass through the outlets (70) varies depending on arelative height each outlet (70) is from the ground. Such a featurecould be advantageous since it is common for ambient winds to increasein velocity with an increase in distance from the ground.

It should be noted that the present invention may be advantageouslyoperated without use of the differential air flow described above.However, different suction and blowing rates on the upstream side (42)and the downstream side (44) enables the performance of the apparatus tobe maximized throughout a wide range of ambient wind conditions andresultant winds.

As shown in FIGS. 7(a), 7(b), 8(a), and 10(a)-(b), power is provided tothe boundary layer control system and the microprocessor control systemby a mechanical power supply system. The mechanical power supply systemincludes an aerodynamic fairing (148), and a road wheel (150) having anintegral primary drive sprocket (152). The road wheel (150) is mountedin a conventional trailing "A" frame suspension (154) and ismechanically coupled to the blower (72) by two pairs of toothed belts(220). The trailing "A" frame suspension (154) includes mounting plates(156) which attach the power supply system to the bluff body (20),pivots (158) which allow trailing arms (160) to follow the road contour,a stub axle (162) for the road wheel (150), suspension springs (164),and shock absorbers (166). The longitudinal axis of pivots (158)coincides with co-axial drive sprockets (170).

The mechanical power supply system requires no speed regulation becauseits primary mechanical energy is derived from the road surface in directproportion to vehicle speed. As a consequence, the strength of thepressure shell (194) is zero at rest and reaches a maximum strength atmaximum vehicle speed. This simplification offers the distinct advantageof speed regulation. Additionally, the disclosed mechanical power supplysystem offers a clear advantage of safety over power supply systemsusing small gasoline or diesel engines, because no additional fueling orfuel lines are required. The disclosed mechanical power supply system isalso simpler than an electric power supply system because it imposes nospecial electrical generating or wiring requirements on a tractor,trailer or semi-trailer. The energy penalty of the power supply systemis masked by the greater energy savings made possible by aerodynamicimprovement. It should be noted that, although a mechanical system isdescribed, either electrical, hydraulic, pneumatic or mechanical means,or their variants, singly or in combination, may be used for the powersupply system without departing from the spirit of the invention.

A thrust augmentation system is shown in FIGS. 6(a)-8(e) and11(a)-13(d). The thrust augmentation system includes either static flowcontrollers (82) or dynamic flow controllers (84) attached to theperiphery of the bluff body (20) at the rear end or trailing edgethereof. The controllers (82,84) are designed to utilize air passingthrough air outlets (70) to greatly augment and organize the flow behindthe bluff body trailing edge (22), and to modify and reenergize the basepressure (26). A variable portion of the boundary layer flow (65) iscaptured by the boundary layer control system as directed by themicroprocessor (74). The variable portion so captured is re-injected asa primary flow (86) into an otherwise turbulent wake (28). The primaryflow (86) is supplemented by the entrainment of considerable additionalmass from the remaining boundary layer to serve as a secondary flow(88), and will be explained hereinbelow. The re-injection providesintense organization of the injected primary flow (86) and entrainedsecondary flow (88) by action of the thrust augmentation system.

FIGS. 11(f) and (g) illustrate one embodiment of a static flowcontroller (82) in the form of a flow directing device (100) of highaspect-ratio and of a length equal to the height or width of the bluffbody (20). The flow directing device includes a pair of flow directorsor flow diverters in the form of thin airfoils (104). A plurality of theoutlets (70) are arranged between the flow directors to direct theprimary flow (86) between the flow directors. Flow conditioners (114)may be utilized in the slotted area to further condition the primaryflow ejected through air outlets (70) into the turbulent wake (28), asshown in FIGS. 7(c) and 11(g).

The flow directors may include two thin aerodynamic "flat plates" (102)as shown in FIG. 11(c), two thin airfoils (104) as shown in FIG. 11(f),or combinations of flat plates (102) and airfoils (104) as shown inFIGS. 11(d)-(e). The longitudinal sides of the flow directors may havetheir chord-wise axes (106) parallel as shown in the upper portion ofFIG. 11(a) and in FIGS. 11(c)-(f), or intersecting at a convergenceangle φ (108) as shown in the lower portion of FIG. 11(a), in FIG.11(b), and in FIGS. 11(h)-(o). Also, the longitudinal sides of the flowdirectors may have their longitudinal axes (110) on a commonperpendicular from the side of the bluff body (20) as shown in FIGS.11(a)-(o), or offset from the bluff body (20) or each other by an offsetdistance α (112) as shown in FIGS. 11(p)-(s). The exposed surfaces ofthe flow directors may be smooth, ribbed or rough. Operation of thestationary flow controller (82) will be described further hereinbelow.

FIGS. 12(a)-(d) provide a detailed sectional view of a dynamic flowcontroller (84), which includes a circular venturi (118) concentric witha high pressure primary nozzle (120), which is connected to an outlet(70). The nozzle (120) exhibits a compound coning angle μ (122) and asweepback angle Ω (124). The term "high pressure" is relative to thedynamic stagnation pressure (14) of the boundary layer, which may bemeasured as a fraction of an inch of water column, at typical highwayspeeds. The action of a dynamic flow controller (84) may be bestunderstood by following the movement of air through the apparatus. Airfrom the boundary layer flow is drawn into the air inlets (66), throughthe ductwork (68) and the blower (72), and is ejected through the airoutlets (70) as a primary flow (86) into the high pressure primarynozzles (120) centrally located in the circular venturi (118). Theprimary flow (86) may be distributed within the primary nozzle (120)through any number of internal passageways (preferably 3 or 4) whichterminate in final outlet nozzles (121).

The nozzles (120) may be either fixed or rotatable. Rotatable highpressure nozzles (120) may be mounted on bearings (130) fixed to airoutlets (70). The bearings (130) may include ball bearings, etc., orthey may be free-wheeling. The inner periphery of the circular venturi(118) preferably has a cross-section which is an airfoil of "Clark Y"profile, which profile was widely used on the wings of early aircraft.

As the primary flow (86) is introduced into circular venturi (118) bythe outlet nozzles (121), part of its flow energy is transferred to aninduced low-energy secondary flow (88) entrained from the remainingboundary layer at the inner periphery (132) of the circular venturi(118). A net energy and momentum exchange between the high pressureprimary flow (86) and the low pressure induced secondary flow (88) iscompleted before the combined flows (126) are released through the exitplane (128) of the circular venturi (118). FIG. 12(c) provides anupstream view into the dynamic flow controller (84) at exit plane (128).

Located just upstream of and partially surrounding the nozzle (120) is abackflow redirector (134), which acts to convert counterproductiveleakage of the primary flow (86) into beneficial flow. For the thrustaugmenter to work properly, the primary flow must be ejected from thenozzle (120) at a pressure several times greater than the pressure ofthe secondary flow (88). At highway speeds, the dynamic pressure of theboundary layer of a bluff body does not exceed an inch of water column.consequently, the primary flow (86) is only pressurized up to about 8inches of water column (about 0.28 psi). With such a low pressure forthe primary flow (86), the bearing (130) must have exceedingly lowresistance to turning, or the sweepback angle (124) of the outlet nozzle(121) will not provide enough reactive force to turn the nozzle (120).The internal surface of the nozzle (120) is therefore relieved such thatit does not touch the end of the outlet (70) Unfortunately however, thisrelief allows a portion of primary flow (86) to escape upstream, whichinhibits the entrainment of the secondary flow (88). Thus, the backflowredirector (134), by means of a curved inner surface (135) thereof,redirects the leakage back downstream through the circular venturi(118), along an outer surface of the nozzle 120. This redirection notonly solves the problem of counterproductive leakage, but also enhancesthe operation of the nozzle (120), since the nozzle (120) is bathed in athin veil of higher-pressure primary, redirected flow, which reducesdrag on the nozzle (120).

The longitudinal axis (116) of the circular venturi (118) intersects theplane of the longitudinal axis (36) of bluff body (20) at a convergenceangle φ (108) of approximately 10 to 15 degrees. Convergence angle φ(108) is chosen to approximate the convergence of a conventionalstreamlined body (2) from its widest point to its trailing edge (6).Specifically, the convergence angle (108) is chosen to be an angle whosetangent equals one-half the maximum width of the bluff body (20),divided by the distance along a longitudinal axis of a streamlined bodyfrom its maximum width to its trailing edge. The convergence angle (108)is thus selected to define a "pressure shell" which approximates asclosely as possible a streamlined body. This approximation reduces thechance of early separation of the boundary layer and thus reduces drag.

FIG. 12(b) is taken along section line B--B of FIG. 12(a) through thelongitudinal axis (116), and illustrates compound coning angle μ (122).FIG. 12(d) is taken along section line D--D of FIG. 12(b) through thelongitudinal axis of final outlet nozzle (121) to illustrate sweepbackangle Ω (124). Compound coning angle μ (122) and sweepback angle Ω (124)are chosen to maximize the performance of the dynamic flow controller(84).

At the exit plane (128) of a controller such as the dynamic flowcontroller (84) shown in FIG. 12(a), the primary flow (86) and thesecondary flow (88) are mixed with a mutual energy exchange, and areexhausted as combined flow (126). The amount of energy exchanged is suchthat the velocities of the two flows are about equal when passingthrough the exit plane (128). For equal velocities to exist at exitplane (128), it is believed that the sum of the kinetic inflow energiesmust equal the sum of the kinetic outflow energies, as predicted by J.V. Foa in his text Elements of Flight Propulsion, John Wiley & Sons,1960, pp. 234, which is hereby incorporated herein by reference. Inother words, air outlet (70) is used innovatively to provide primary airflow to a thrust augmenter as a "trigger" to induce the movement of amuch greater volume of air as entrained secondary flow (88).

Combined flow (126), whether produced by dynamic flow controllers (84)or static flow controllers (82), exhibits organization and intensevorticity as it progresses past bluff trailing edge (22), creating adownstream-extending small high-energy vortex (90). A plurality ofstatic flow controllers (82) or dynamic flow controllers (84) arepositioned around the periphery of the bluff body trailing edge (22)such that a series of mutually-supporting small vortices (90) of thesame sign of rotation are formed, and then roll up into a sheet vortex(92). The creation of the sheet vortex (92) redefines base pressuredistribution because it is apparent to the remaining boundary layer asan invisible streamlined after-body (94) or "boat-tail".

FIGS. 13(a)-(d) show the organization of flow and the creation of a"pressure shell" as an invisible streamlined afterbody (94) at the rearof the bluff body (20). The pressure shell (94) is created from theplurality of small individual high-energy vortices (90) of the same signof rotation which are automatically created by the controllers (82,84).The small vortices (90) combine to form a sheet vortex (92) to replacethe turbulent wake (28) of bluff body (20). Streamlines (8) of FIG.13(d) indicate the flow of the remaining boundary layer around a bluffbody which has an invisible streamlined afterbody (94). As in FIG. 1(a),the streamlines of FIG. 13(d) indicate a more complete pressure recoveryat the trailing edge (6) as the base pressure (24) is increased,preferably to more nearly match (be generally near) the dynamicstagnation pressure (14) at the leading edge (4) of the bluff body (20).

The benefits of the thrust augmentation system working in conjunctionwith the boundary layer control system are three-fold. First, boundarylayer control reduces the broad eddying wake that would otherwise formdownstream of the widest point (21) of bluff body (20) and this reducesaerodynamic drag. Second, considerable additional mass derived from theremaining boundary layer is entrained as secondary flow (88) andreleased behind the bluff body (20) to provide actual, but modest thrustaugmentation. Thirdly, a disproportionately large aerodynamic benefit isachieved by the combination of a plurality of mutually supportingvortices to form a single large vortex with structure and organizationderived from combined flows (126) so as to replace an otherwise chaoticand turbulent wake (28) with an invisible streamlined after-body (94) asdescribed above. All three benefits combine to increase base pressure,reduce aerodynamic drag and increase fuel efficiency.

Since the invisible after-body (94) is a "pressure shell" derived fromcontrolling and organizing the flow of the original boundary layer,there is no physical structure to significantly alter dimensions of thebluff body (20), to inhibit loading and unloading operations, to reducecargo capacity, to require substantially greater parking space or toalter current shipping practice.

Also, static flow controllers (82) or dynamic flow controllers (84) maybe readily and permanently mounted to the rear of a trailer equippedwith "roll-up" overhead doors. For trailers fitted with conventionalhinged rear doors, retrofit offset hinges may readily be installed toallow permanent attachment of fixed flow controllers (82) or (84).Alternatively, flow controllers (82) or (84) may be mounted at the rearof the bluff body (20) such that they can be swung or moved forwardly,toward the front of the bluff body (20) so as to allow full utilizationof hinged rear doors, without departing from the spirit of theinvention.

It is to be noted that, while extended reference has been madehereinabove to the use of the method and apparatus of the presentinvention with a bluff body such as a tractor-trailer, the method andapparatus may equally well be used with other bluff bodies such as vans,campers, etc., or with semi-streamlined vehicles such as automobiles.

What is claimed is:
 1. A method of reducing drag produced by relativeair movement on a moving, non-lifting bluff body, said body having afront end and a rear end, wherein said body has a substantially closedsurface at its rear end, wherein said body is an entire moving system,comprising of steps of:modifying a rear air pressure behind a rear ofsaid body such that said rear air pressure is increased, said airpressure modifying step including inletting an amount of air from aboundary flow around said body, and forming a pressure shell behind saidrear of said body, said pressure shell forming step including forming alarge vortex behind said rear of said body by outletting said inlet airat said rear of said body in a plurality of small high-energy vorticesof one sense of rotation, which combine to titan a large vortex of onesense of rotation behind said entire moving system.
 2. A method asclaimed in claim 1, wherein said air pressure modifying step furtherincludes sucking air from inlets into ducts, and blowing said inlet airfrom said ducts toward outlets.
 3. A method as claimed in claim 1,wherein said air pressure modifying step further includes sensing apressure and yaw angle of said relative air movement, and controlling arate at which said air is inlet and a rate at which said inlet air isoutlet based on said sensed pressure and yaw angle of said relative airmovement.
 4. A method as claimed in claim 3, wherein said outlettingstep includes outletting said inlet air around a periphery of said rearof said body, and wherein said controlling step includes controlling anamount of said inlet air which is outlet at different locations on saidperiphery.
 5. A method as claimed in claim 3, wherein said sensing stepincludes sensing air pressures produced by said relative air movement onsides of said body.
 6. A method as claimed in claim 1, wherein saidoutletting step includes outletting said inlet air at a plurality oflocations which are spaced around a periphery of said rear of said bodysuch that said plurality of small high-energy vortices form a singlelarge vortex.
 7. A method as claimed in claim 1, wherein said inlettingstep includes inletting a portion of said boundary flow around forwardportions of said body.
 8. A method as claimed in claim 1, wherein saidoutletting step includes entraining a portion of said boundary flow withsaid inlet air as said inlet air is outlet.
 9. A method as claimed inclaim 1, wherein said outletting step includes holding an outletstationary relative to said body, and passing said inlet air throughsaid outlet.
 10. A method as claimed in claim 1, wherein said outlettingstep includes passing said inlet air through an outlet which is rotatingrelative to said body.
 11. A method as claimed in claim 1, wherein saidoutletting step includes outletting said inlet air at a number oflocations spaced around a periphery of said rear of said body, indirections angled toward a center of said rear.
 12. An apparatus forreducing drag on a moving body, comprising:a controller; at least onesensor for sensing a pressure and yaw angle of a relative air movement,an output of said sensor being connected to said controller; at leastone air inlet for inletting air; a plurality of thrust augmentersconnected to said air inlet; and at least one valve connected betweensaid air inlet and said plurality of thrust augmenters, said controllerbeing connected to said valve to control a rate of air flow through saidvalve in response to said output of said sensor.
 13. An apparatus asclaimed in claim 12, further including:a blower connected between saidair inlet and said plurality of thrust augmenters to suck air into saidair inlet and to force inlet air out of said plurality of thrustaugmenters.
 14. An apparatus as claimed in claim 12, wherein saidapparatus includes a plurality of air inlets and a plurality of valvesconnected to said controller, a valve being connected between each ofsaid air inlets and said plurality of thrust augmenters such that saidcontroller controls a rate of air flow from each of said air inletsbased on said output of said sensor.
 15. An apparatus as claimed inclaim 12, wherein said apparatus includes a pair of sensors for sensinga pressure and yaw angle of a relative air movement at spaced locations.16. An apparatus as claimed in claim 12, wherein said apparatus includesat least two valves connected to said controller, each of said valvesbeing connected between said air inlet and a set of said thrustaugmenters such that said controller controls a rate of air flow througheach set of thrust augmenters separately, based on said output of saidsensor.
 17. An apparatus as claimed in claim 12, wherein said apparatusincludes valves connected to said controller, and connected between saidair inlet and said thrust augmenters, a rate of air flow through each ofsaid thrust augmenters being controllable by said controller throughsaid valves based on a relative height of each said thrust augmenter.18. An apparatus as claimed in claim 12, wherein each of said thrustaugmenters includes a flow directing device having a pair of flowdirectors, and an air outlet which is connected to said air inlet andwhich is held stationary between said flow directors, said flowdirectors entraining a portion of a boundary layer air flow with airflowing from said air outlet.
 19. An apparatus as claimed in claim 18,wherein at least one of each of said pair of flow directors is a flatplate.
 20. An apparatus as claimed in claim 18, wherein at least one ofeach of said pair of flow directors is an airfoil.
 21. An apparatus asclaimed in claim 12, wherein each of said thrust augmenters includes acircular venturi, an air outlet which is connected to said air inlet andwhich is mounted rotatably within said venturi, and a backflowredirector mounted at a rear end of said air outlet.
 22. An apparatus asclaimed in claim 21, wherein each of said backflow redirectors is heldstationary relative to a respective venturi, and is mounted around anupstream end of a respective air outlet to direct backflow of air fromthe air outlet downstream through the venturi.
 23. An apparatus forreducing drag on a moving non-lifting bluff body, said body having afront end and a rear end, wherein said body has a substantially closedsurface at its rear end, wherein said body is an entire moving system,comprising:means for inletting an amount of air from a boundary layerflow; and means for forming a pressure shell, said pressure shellforming means including means for forming a plurality of smallhigh-energy vortices of the same sense of rotation, which combine toform a large vortex of one sense of rotation behind said entire movingsystem, said small vortex forming means including means for outlettingsaid inlet air, and means for entraining a portion of the boundarylawyer flow with said inlet air.
 24. An apparatus as claimed in claim23, further including:sensing means for sensing a pressure and a yawangle of a relative air movement; and controlling means for receivingsaid output of said sensing means and for controlling a rate of air flowthrough said outletting means based on said output.
 25. An apparatus asclaimed in claim 24, further including:valve means connected in a flowpath between said inletting means and said outletting means, saidcontrolling means controlling said valve means to thereby control therate of air flow through said outputting means.
 26. An apparatus asclaimed in claim 24, wherein said outletting means includes a pluralityof spaced outlets, and wherein said controlling means controls a rate ofair flow through said outlets based on a relative lateral and verticalposition thereof.
 27. An apparatus as claimed in claim 23, furtherincluding:blowing means for sucking air into said inletting means andfor forcing inlet air out of said outletting means.
 28. An apparatus asclaimed in claim 23, further including:backflow redirecting meanslocated at an upstream end of said outletting means for redirectingbackflow from said outletting means downstream through said entrainingmeans.
 29. An apparatus as claimed in claim 23, wherein said entrainingmeans includes a flow directing device having a pair of spaced flowdirectors, and said outletting means includes an outlet held stationarybetween said flow directors, at least one of said flow directors beingan elongated flat plate.
 30. An apparatus as claimed in claim 23,wherein said entraining means includes a flow directing device having apair of spaced flow directors and said outletting means includes anoutlet held stationary between said flow directors, at least one of saidflow directors being an airfoil.
 31. An apparatus as claimed in claim23, wherein said entraining means includes a circular venturi and saidoutletting means includes an outlet mounted rotatably within saidventuri.
 32. An apparatus as claimed in claim 23, wherein saidentraining means includes a circular venturi and said outletting meansincludes an outlet held stationary within said venturi.
 33. An apparatusas claimed in claim 31, wherein an inner periphery of said circularventuri has a cross-section which is an airfoil of "Clark Y" profile.34. An apparatus for reducing drag on a moving body, comprising:at leastone inlet mounted on a forward portion of said body; a plurality ofthrust augmenters mounted on a rear portion of said body; at least oneduct mounted on said body and connecting said inlet with said thrustaugmenters, at least one valve in said duct; a pair of sensors mountedon said body to sense a pressure and a yaw angle of a relative airmovement on said body; and a controller connected to said sensors andsaid valve to control said valve and thus an air flow rate through saidthrust augmenters, based on outputs of said sensors.
 35. An apparatus asclaimed in claim 34, further including:a blower located in said duct,said blower being connected to a rotatable wheel mounted below saidbody.
 36. An apparatus as claimed in claim 34, wherein each of saidthrust augmenters include a flow diverting device having a pair ofspaced flow diverters fixed to said body, and an outlet fixed to saidbody between said flow diverters.
 37. An apparatus as claimed in claim36, wherein at least one of each of said pairs of flow diverters is aflat plate.
 38. An apparatus as claimed in claim 36, wherein at leastone of each of said pairs of flow diverters is an airfoil.
 39. Anapparatus as claimed in claim 34, wherein each of said thrust augmentersincludes a circular venturi fixed to said body, and an outlet rotatablymounted to said body within said venturi.
 40. An apparatus as claimed inclaim 39, wherein each of said thrust augmenters further includes abackflow redirector mounted around an upstream portion of said outlet toredirect backflow of air from said outlet downstream through saidventuri.
 41. An apparatus as claimed in claim 34, wherein each of saidthrust augmenters includes a circular venturi fixed to said body, and anoutlet held stationary within said venturi.
 42. An apparatus as claimedin claim 34, wherein said apparatus includes at least a pair of inletsmounted on opposite sides of forward portions of said body, and whereinsaid thrust augmenters are mounted around a periphery of a rear of saidbody.
 43. An apparatus as claimed in claim 34, wherein said thrustaugmenters are angled toward a center of a rear of said body.
 44. Anapparatus as claimed in claim 34, wherein said body is a truck.
 45. Anapparatus as claimed in claim 34, wherein said body is an automobile.46. An apparatus as claimed in claim 34, wherein said body is a van. 47.A thrust augmenter, comprising:a circular venturi; a nozzle mountedrotatably within said venturi; and a backflow redirector mounted aroundan upstream end of said nozzle, said backflow redirector including meansfor redirecting upstream-directed leakage from said nozzle downstreamaround an outer surface of said nozzle.
 48. A thrust augmenter asclaimed in claim 47, wherein an inner periphery of said venturi has across-section which is an airfoil of "Clark Y" profile.